1. Field of the Invention
The invention relates to air-operated diaphragm brakes for vehicles and particularly to combination service and spring brake actuator assemblies.
2. State of the Prior Art
An air brake system for a vehicle such as a bus, truck or the like typically includes a brake shoe and drum assembly which is actuated by means of an actuator assembly operated by the selective application of compressed air. Conventional air brake actuators have both a service brake actuator for actuating the brakes under normal driving conditions by the application of compressed air and a spring-type emergency brake actuator which causes actuation of the brakes when air pressure has been released. The emergency brake actuator includes a strong compression spring which forces application of the brake when air is released. This is often referred to as the spring brake.
Typically, the spring brake actuator is disposed in tandem with the service brake actuator. When full pressure is applied to the spring brake actuator, air pressure acting against a diaphragm compresses the compression spring. A spring brake actuator rod, operably connected between the diaphragm and the service brake actuator does not affect the operation of the service brake. When the brake is to be applied during normal driving operation, compressed air is provided to the service brake actuator which, acting against a diaphragm, causes a service brake push rod to be extended and causes the brakes to be applied with an application force which is proportional to the air pressure applied to the service brake actuator. In the event of a loss of air pressure or an intentional exhaustion of air from the spring brake actuator, the brake will be mechanically activated by the force of the compression spring acting on the spring brake actuator rod which in turn acts upon the service brake push rod to apply the brakes. Thus, the spring brake portion serves both as a parking brake and an emergency brake.
In a typical prior art air brake system, the spring brake actuator and the service brake actuator are disposed in a single housing comprising a spring brake portion and a service brake portion. The service brake portion includes an air chamber partially defined by a flexible service diaphragm acting against a service brake push rod and a return spring to assure proper release of the brake when air is exhausted from the air chamber. The spring brake portion includes a spring chamber and an air chamber, both partially defined by a spring brake diaphragm acting against a spring pressure plate to compress the compression spring in the spring chamber when air pressure is applied to the spring brake diaphragm in the air chamber. In some applications, the spring brake actuator rod extends through the spring brake diaphragm to be received by a tubular recess in the pressure plate, thereby integrally connecting to the pressure plate.
In operation, the spring brake actuator rod is pushed outwardly from the air chamber through a housing opening and bearing provided with a pneumatic seal to engage the service diaphragm and push rod of the service brake, thereby causing the brake to be applied. The spring brake diaphragm is provided with a centrally disposed aperture having an annular edge and the actuator rod extends through the opening and engages the annular edge to form an airtight seal against the tubular recess in the pressure plate. The actuator rod is hollow with a central bore and includes a brake releasing cage bolt which extends into the central bore. An end plate on the caging bolt engages the spring brake pressure plate to draw the spring to a compressed state when the spring is caged by rotation of the bolt.
Prior art brake assemblies suffer from several problems including the multiplicity of parts, the cost of the various parts of the brake assembly, and the various assembly steps required in manufacture of the brake assembly. Simplifying the method of manufacture and increasing the reliability of the product, through reducing the number of parts or increasing the quality of the parts employed, are overriding goals of brake assembly design.
The conventional spring brake assembly, as detailed above, includes a spring brake actuator rod formed integral with a spring brake pressure plate and secured to the spring brake diaphragm. The conventional method of securing a spring brake diaphragm with a centrally-disposed aperture having an annular edge requires sealing the annular edge of the diaphragm to the actuator rod where it extends through the diaphragm aperture to form an airtight seal. More specifically, the annular edge of the centrally-disposed aperture opening is clamped between a portion of the tubular recess in the pressure plate and the actuating rod to form the airtight seal. The tubular recess of the pressure plate is provided with an expanded portion for accommodating the collar of the diaphragm and further pressing the collar against the outside surface of the actuating rod to maintain an airtight attachment between the actuating rod and the diaphragm. This is the structure disclosed by Bowyer, U.S. Pat. No. 5,105,727.
While the aforementioned design provided several advantages tending to reduce the cost of manufacture, including inside diameter retention, pressure seal of the diaphragm, and rigid attachment of the actuating rod to the pressure plate, the design required a groove in the tubular recess of the pressure plate for receiving the annular edge of the diaphragm, which is costly to produce. Also, the design did not preserve valuable surface area within the pressure plate's tubular recess, which is circumferentially press-fit on the actuating rod. This design results in the need for a larger tubular recess for receiving and securing the actuating rod because its surface area is partially consumed by the internal groove accommodating the annular edge of the diaphragm. Moreover, replacement of the diaphragm is expensive and time consuming because it requires removal of the actuating rod from the pressure plate's tubular recess, which are in press-fit connection.